Petroleum residues, as is well known, may contain significant quantities of vanadium which it may be desirable to recover, whether for the value of the vanadium or to avoid environmental pollution and indeed such residues can provide a valuable source of vanadium for the production of various vanadium compounds.
Such residues are generally solid residues of cracking, refining combustion or gasificatiion processes and generally is in the form of a high vanadium refractory solid composition.
It is known to recover vanadium compounds from such residues by mechanically comminuting the residue and treating the comminuted material with alkali compounds by a heating process in a heating furnace having an oxidizing atmosphere. The material which results from this treatment can be leached or extracted with water and vanadium compounds recovered from the leaching liquor which thus results. The vanadium containing residues can contain 10-60% by weight V.sub.2 O.sub.5, 3-15% by weight nickel and 4-40% by weight MgO, as well as substantial quantities of carbon, sulfur, calcium and silicon.
The customary process used heretofore carries out the heating in the form of a roasting. The product which is withdrawn from the roasting furnace contains significant amounts of impurities which preclude the effective refinement of the product to a useful vanadium compound, i.e. precludes the recovery of a potassium metavanadate, vanadium pentoxide or vanadyl oxalate from the roasted composition.
Indeed, the production of pure vanadium compounds and especially these compounds are useful in the chemical industry, for example in the production of catalysts for sulfuric acid, phthalic acid and maleic acid synthesis, are useful in the ceramics industry for the generation of dyestuffs, pigments, coloring agents and enamels, and are useful for various metallurgical applications where high purity vanadium compounds are advantageous as, for example, where vanadium pentoxide is used in alloying in steel, e.g. for ferro alloys.
In order to supply such needs of high purity vanadium compounds, a technology has been developed in which vanadium raw materials such as titanomagnetite slag is used and is subjected to a number of relatively expensive and time-consuming purification steps.
In this technology and in other vanadium compound production systems, in which a multiplicity of stages are carried out, waste water is generated which represents a burden upon the environment even if there has been complete or practically complete removal of heavy metals from the waste water. The waste water continues to contain large quantities of neutral salts which are disadvantageous when discharged into bodies of water because of the overall increase in the salinity even where no added danger is provided by heavy metal discharge.
At the same time as the disadvantages of recovering vanadium and its compounds from metallurgical products have become increasingly significant, the accumulation of vanadium-containing wastes from the treatment and combustion or firing of petroleum has become a problem. Indeed, the processing of Central America crudes especially have given rise to environmental problems because of the high heavy metal and especially vanadium content of the petroleum processing residues.
Landfill disposal of these residues is not possible without pretreatment because at least in part the residues can be leached by ground water to release water-soluble heavy metal compounds.
Indeed, the chemical and ceramic industries, as well as specialty metallurgical applications require certain vanadium compounds, namely, ammonium polyvanadate in moist or dry form, sodium ammonium vanadate as wet cake, and ammonium metavanadate, potassium metavanadate, vanadium pentoxide and vanadyloxalate as solutions.
In the past, such compounds had to be fabricated from such known raw materials as titanomagnetites or the slags thereof are required generally the following sequence of steps:
comminution and milling of the slag; PA1 alkali oxidizing roasting; PA1 leaching out of the sodium vanadate thus produced; PA1 purification to separate out, for example SiO.sub. 2 and Fe; PA1 precipitating out sodium-ammonium vanadate (SAV); PA1 solubilizing the SAV and precipitating out ammonium polyvanadate (APV); and PA1 solubilizing the APV and precipitating out NH.sub.4 VO.sub.3.
All in all, therefore, the processes involved in producing useful vanadium compounds in accordance with prior art technology were inordinately complex and expensive to carry out and did not and are not able to eliminate the drawbacks of high production of waste water and accumulation of petroleum treatment residues with high heavy metal content.
In the conventional processes for the handling of vanadium-containing crude oils, whether by coking, demetallizing, cracking, gasification or combustion, vanadium-containing solid residues are obtained which cannot, as has already been noted be disposed of in landfills or the like.
Conventional processes for treating such residues have included the following: mixing the vanadium-containing solid residues with vanadium oxides and subjecting them to alumino-thermic reduction in electric furnaces to recover FeV (ferrovanadium). This product, however, because of its high content of S, C, Ni, Si and Mg, is considered to be of low grade and second quality so that direct use in metallurgical processes is frequently counterindicated.
The vanadium residue can be subjected to acid treatment with subsequent separation of the vanadium by precipitation or extraction. The problem with this is that a strongly acid waste water is obtained which, because of its heavy metal and salt content, i.e. its content of nickel, iron, aluminum and magnesium sulfate, may be as detrimental to the environment as landfills of the residue or even more detrimental. In any event, the cost of treating such waste water is prohibitive.
The vanadium residues have also been cut with titanomagnetites and slag and have been subjected to roasting utilizing the process previously described for the recovery of vanadium from metallurgical residues. This, however, requires the substantial dilution of the high vanadium-containing residues for roasting since with vanadium content in excess of 7% in the residue, the alkali salt treatment is not effective.